1. Field of the Invention
This invention relates to control of biofouling in oil and gas wells and facilities caused by sulfate reducing prokaryotes. More specifically, it relates to control of prokaryote caused souring, fouling and corrosion by reduction of problematic prokaryotes with naturally occurring lysing organisms, particularly sulfate-reducing prokaryotes by proliferating suitable virulent lysing organisms under conditions in which problematic prokaryotes thrive, including in a gas production wellbore.
2. Background
Microbial fouling is a serious problem in the oil and gas industry. Microbial-evolved hydrogen sulfide sours oil and gas reservoirs, elevates risk and devalues the product. Microbial produced iron sulfide production creates black powder accumulation, causing piping, and pipeline blockages. Microbial-influenced corrosion attacks the whole oil and gas system from fracture tank to refinery. Microbes also degrade fracture fluid additives.
These problems are especially acute in shale gas operations. Gas-containing shale geological formations such as the Barnett Shale in Texas and the Marcellus Shale in the eastern US require use of large-volume hydraulic fracturing technologies.
In a typical hydrofracture operation, 11-19 million liters (2.9 to 5 million gallons) of water is collected in large purpose-dug ponds (“frac ponds”) from a variety of sources; aquifers, chlorinated city water, pond, river and lake water. Each of these water sources have some level of indigenous prokaryote microorganism (bacterial and archaeal) populations that will maintain activity in active viable numbers, and will accumulate during the ponds' period of open exposure. Further, water recovered from previous hydro-fracture operations, “flow-back” or “produced” water, is typically re-used by mixing with this frac pond water. If the water is not treated correctly, recycling can lead to “black water”, scaling, souring, and MIC (microbiologically influenced corrosion).
Sulfate reducing prokaryotes endemic to or introduced into the formation will encounter favorable growth conditions during the hydrofracture operation. Upon completion some gas wells are “shut in” while surface processing equipment and flow-lines are installed, leaving time for microorganisms to colonize. Ultimately, viable microorganisms within biofilms can produce the sulfide necessary to sour a sweet gas reservoir, and contaminate flow lines, water tanks, and disposal facilities. Aside from the extreme costs of reservoir souring, many tight shale gas productions in the US are solely dependent on the ability to treat and dispose of the flowback (produced) water. Therefore, the water either utilized or produced, is often key to the commercial viability of the gas developments.
To counter bacterial fouling, reservoir souring, and to “clean” water for disposal broad spectrum chemical biocides are used. For a biocide to work, it must diffuse and kill at rates faster than the growth rate of bacteria. If biocides are unable to do so then prokaryote microorganisms grow within the pores of the reservoir formation, in biofilms which bud off inoculums to contaminate downstream through laterals, tubulars, and mobile processing equipment. With the porosity of the formation rock at nanodarcy size, developing biofilm can easily choke off the well, ultimately affecting the conductivity and thus productivity of the gas well.
These broad spectrum chemicals cost the oil industry over $200 million annually, while the cost of “corrosion” to oil upstream production and gathering systems, flowlines, and liquid transmission pipelines is estimated at $7 billion annually in the US alone. Tetrakis (hydromethyl) phosphonium sulfate (THPS) and hypochlorite bleach are the most commonly used antimicrobials in the Barnett shale operations area and cost approximately $50,000 each hydrofracing operation. Customary biocides include glutaraldehyde, glutaraldehyde/quaternary ammonium compound blends, isothiazolin, tetrakis(hydromethyl)phosphonium sulfate (THPS), 2,2-dibromo-3-nitrilopropionamide, and bronopol. However these biocides often have major health risks to humans and all animals in the food chain.
It is uncertain whether the currently used biocides are even effective against sulfate reducing archaea. Further compounding the issue of toxicity, many of the biocides within hydrofracing systems have proven to be less than effective as numerous turnkey gas wells have become sour, and many disposal wells and horizons are being quickly plugged. Flow back water recycling is being reduced, and water shortages and fouling issues are threatening to curtail exploration and production in tight gas shale areas.
The scale of the problem is enormous. The Barnett Shale extends over 5,000 square miles in north central Texas. A total of 6,519 gas wells with a further 4,051 permitted locations existed as of Aug. 15, 2007. Wells are being drilled within populated areas such as the Dallas-Fort Worth city limits where it is vital to minimize risk and environmental impact.
EPA registered biocides cannot be introduced into open ponds as they will permeate into the groundwater, killing aquatic organisms and ultimately be consumed by terrestrial animals, and possibly humans. Since biocides may remain in residual flowback and produced water, this water constitutes a waste handling and disposal issue. Overall, biocide usage in the petroleum industry is facing growing regulatory resistance due to its negative impact on the environment and associated health risks.
Another problem with biocide use is in assessing their effectiveness. In typical biocide assessment practices, samples of hydrofracture water are diluted and cultured in specialized growth medium under various conditions, with and without biocide, for various lengths of time and then compared for bacterial cell density, resulting in more than 40 test cultures each time. There are seasonal variations in bacteria, requiring different growth and test conditions, to which the bacteria may respond differentially. The results take days and thus cannot be used for optimization of biocide application. The typical field solution to this uncertainty is to apply massively excessive concentrations of sodium hypochlorite (Use of Microbiocides in Barnett Shale Gas Well Fracturing Fluids to Control Bacteria Related Problems; J. K. Fisher, K. Johnson, K. French and R. Oden, Paper 08658, NACE, International; 2008 Corrosion Conference and Expo).
Moreover, these field tests vastly underestimate the variety, type and amount of sulfate reducing microorganism that are actually present in the water and that are present in the wellbore (Larsen, Soresen, Hojris and Shovas; Significance of Troublesome Sulfate-Reducing Prokaryotes (SRP) in Oil Field Systems; Paper 09389, NACE Corrosion 2009 Conference and Expo).                “Sulfide generation by sulfate-reducing prokaryotes (SRP) is the major cause of reservoir souring ad microbiologically influenced corrosion (MIC). The monitoring of SRP in oil fields is typically carried out by cultivation based methods. It is widely accepted that the cultivation approach grossly underestimates population sizes by several orders of magnitude due to the majority of SRP in oil field samples being not readily viable in selective culture media.”        “Only a small fraction (usually less than 1%) of the microorganisms in a sample will grow in enrichment media in the laboratory. Nevertheless, monitoring of microbiological sulfide production in relation to souring and MIC in the oil industry still rely largely on cultivation-based techniques such as the most probable number (MPN) technique, potentially resulting in severe misinterpretation of the actual system condition.”        As an example, it has become clear that sulfide is not only produced by sulfate-reducing bacteria (SRB), but also by a group of Archaea (sulfate-reducing Archaea, SRA), methanogens and even fermentative microorganisms in the oil field system system.”        
In addition to oil and gas wells that are hydrofractured, other reservoirs are “flooded” with water to enhance oil recovery. In flooding, water is pumped into an injection well to push the oil and/or gas through a formation into “recovery” well(s) in the same field. Since water is injected into the reservoir and it is also contaminated with the same type organisms as the water for hydrofracturing, the same problems of souring, fouling and corrosion occur.
A better control strategy would be: inexpensive to manufacture, environmentally benign, able to evolve with the microorganisms and thus prevent resistance, be targeted towards those microorganisms that constitute the threat and be able to penetrate and destroy biofilms. Such a control strategy would also, optionally, be able to sense and adjust to the different concentrations of microorganisms encountered, even within the well. The present invention is just such a strategy based on bacteriophage or archaeal viruses, the natural predators of prokaryotes (bacteria and archae). Bacteriophage are used as an example to illustrate this invention.